Coal water fuel has the potential to replace fuel oil. More particularly, in the future it is expected that high oil prices will make coal slurry fuels more competitive with and even preferable to foreign oil.
The majority of coal water fuel suppliers use a benefication technique to produce a low ash product. Typically, either physical or chemical coal cleaning techniques are utilized. Generally, however, physical coal cleaning techniques such as froth flotation or oil agglomeration are more economical than chemical leaching techniques. Of these two physical coal cleaning techniques, froth flotation appears to be the most economical and well established.
Froth flotation was first discovered in 1906. It was developed for the non-ferrous minerals industry to recover extremely fine, free minerals from slime. This technique, developed nearly 80 years ago, remains basically the same today. The froth flotation mechanism employs the principles of colloid chemistry, crystallography and physics. Separation of one mineral from another is achieved by the use of specific reagents and chemical conditions. The addition of chemical reagents makes one mineral surface hydrophobic through adsorption, while leaving the other mineral surfaces hydrophilic. The benefication is accomplished by sparging air through the suspension, whereby air bubbles laden with hydrophobic particles rise to the surface of the pulp or slurry, leaving behind the hydrophilic particles.
Froth flotation is a complex physico-chemico-mechanical process. The process and, particularly, bubble-particle attachment is influenced by many variables including pH, pulp or slurry density, particle size, bubble size and air flow.
There are two mechanisms proposed for explaining bubble-particle attachment in flotation. These mechanisms are (1) direct attachment by collision of particle and bubble and (2) precipitation of dissolved gas on hydrophobic particle surfaces. The collision mechanism has been verified using high speed photography and theoretical equations. The gas precipitation mechanism also has been demonstrated. Both mechanisms have attained worldwide recognition, however, the collision mechanism has been universally accepted over the bubble precipitation mechanism.
An example of a basic froth flotation apparatus is disclosed in U.S. Pat. No. 3,339,730 to Boutin et al. The Boutin froth flotation apparatus includes a column having upper, intermediate and lower sections. Initially, ground coal, oil, water and the necessary conditioning and flotation agents are added to and agitated in a conditioning tank. After conditioning, the resulting coal slurry or pulp is fed into the intermediate section of the column. A porous metal air diffuser is positioned in the bottom section of the column to provide air bubbles for floating the hydrophobic material which is then recovered through overflow from the top section of the column.
While relatively effective in separating coal from ash or tailings, the column of the type disclosed in the Boutin et al. patent may still be the subject of improvement to provide still more efficient and effective separation and enhanced recovery as well as grade. More particularly, the Boutin et al. apparatus essentially relies solely upon direct attachment by collision of particle and bubble for flotation. This mechanism is only effective for particles falling within a particular size fraction or range. Other, relatively smaller coal particles on the order of 21 .mu.M are not effectively recovered. Accordingly, there is a significant loss of recoverable coal.
In an effort to address this shortcoming, flotation columns have also been equipped with packing or other structures specifically adapted to define a large number of flow passages extending in a circuitous or tortuous path between the upper and lower portions of the column. For example, U.S. Pat. No. 4,592,834 to Yang, discloses such a column wherein the packing consists of a plurality of spaced plates including uniformly spaced rows of corrugations. These spaced plates are specifically adapted to break individual bubbles into a number of bubbles of smaller size in an effort to increase the size fraction or range of coal with which the bubbles will interact so as to improve overall recovery. While relatively effective, froth flotation columns equipped with such packing may still be improved to provide yet more efficient and effective separation and enhanced recovery.